Electric machines may be driven by inverters having a plurality of solid-state switching devices. Measurement of a switching device's performance requires multiple voltage probes attached to predetermined locations on the switching device. Because other nonintrusive measurement methods (such as Rogwoski coil) have offset and delay problems, resistive shunts are placed in current flow paths to indirectly measure current flow. The probes are used to measure small voltages across the resistive shunts and other key locations on the switching devices. The measurement of these small voltages can be influenced by the effect of ground loops. Ground loops may increase measurement errors and impose ambiguity among the voltage measurements.